Some pharmaceuticals at the stage of being administered to patients have a structure different from that of a compound having actual drug efficacy. After such pharmaceuticals have been administered to a patient, they are metabolized in the living body. As a result, their structure is changed, and they exhibit their drug efficacy at that time. Such a compound before being metabolized in the living body is called a prodrug. Various types of enzymes that metabolize a prodrug to a compound having drug efficacy have been known.
Some amino alcohol derivatives are phosphorylated in vivo and, as a result, exhibit immunosuppressive activity. For example, FTY720 (2-Amino-2-[2-(4-octylphenyl)ethyl]propane-1,3-diol hydrochloride) is phosphorylated in vivo by sphingosine kinase 1 and 2, so that it can be converted to FTY720-phosphate [i.e., (:)2-amino-2 phosphoryloxymethyl-4-(4-octylphenyl)butanol] exhibiting immunosuppressive action (The Journal of Biological Chemistry, (2003), 278, p. 47408-47415).
On the other hand, it is also considered that a compound represented by general formula (I) (wherein each of R1 and R2 represents a hydrogen atom; R3 represents a C1-C6 alkyl group or a hydroxymethyl group; R4 represents a hydrogen atom, a halogen atom or a C1-C6 alkyl group; R5 represents a phenyl group, which is substituted with 1 to 3 substituents selected from the group consisting of a hydrogen atom, a halogen atom, a cyano group, a C1-C6 alkyl group, a C1-C6 alkoxy group, a C3-C6 cycloalkyl group, a halogeno C1-C6 alkyl group, a phenyl group and a benzyloxy group, a halogen atom or a hydrogen atom; X represents a vinylene group (CH═CH group), an oxygen atom, a sulfur atom or a methylamino group; Y represents a single bond, an oxygen atom, a sulfur atom or a carbonyl group; Z represents a single bond or a C1-C8 alkylene group; and n is 2 or 3) is phosphorylated in vivo, so that it can be converted to an active form exhibiting immunosuppressive activity (Japanese Patent Laid-Open No. 2005-46141). However, the mechanism of this phosphorylation in vivo is unknown.
Elucidation of the mechanism of phosphorylation of such compounds has been considered effective for the search for compounds that are activated by being phosphorylated in vivo, for clarification of the mechanism for expression of activity, for selection of a patient who is sensitive to a drug, etc.

It has been reported that a human fructosamine-3-kinase-related protein (which is hereinafter also referred to as “human FN3KRP”) has activity of phosphorylating position 3 of ketosamines such as ribulosamine or psicosamine (Diabetes, (2003), 52, p. 2888-2895). However, the role of such human fructosamine-3-kinase-related protein in vivo is unknown.
As a result of intensive studies directed towards elucidating the phosphorylation mechanism thereof in vivo, the present inventor has found that the protein known as human FN3KRP or human fructosamine-3-kinase (which is hereinafter also referred to as “human FN3K”) is associated with phosphorylation of the compound represented by the aforementioned general formula (I), thereby completing the present invention.
It is an object of the present invention to elucidate an enzyme that phosphorylates in vivo the compound represented by the aforementioned general formula (I), such as (2R)-2-amino-2-methyl-4-[5-(5-phenylpentanoyl)thiophen-2-yl]butan-1-ol. In addition, it is another object of the present invention to provide a method of phosphorylating the aforementioned compound using an enzyme that phosphorylates it. Moreover, it is a further object of the present invention to provide a method of screening for a compound phosphorylated by the aforementioned enzyme. Furthermore, it is a further object of the present invention to provide a method of determining the ability of a subject to phosphorylate a test compound.